We live in the Milky Way galaxy, a huge flat spiral galaxy surrounded by a massive halo of stars and dark matter. The disc of stars, gas and dust in which the Sun resides is 120,000 light-years in diameter; an overwhelming distance on a human scale. In the middle of the disc is the central bulge, a diamond-shaped core of stars.
How did all this structure come together? We know it didn’t all happen all at once, but what were the different chapters in the life of the galaxy? What is the timeline of the Milky Way?
This has been the subject of intense research for decades, but new tools are now online that help target specific structures and stars, helping to understand how the galaxy came to be what it is today. In newly published research, a pair of astronomers tackled this problem and discovered something startling: part of the Milky Way is much older than previously thought, changing the way we think that our cosmic house has been built. [link to paper].
The main observations were made using two facilities. One was ESA’s Gaia satellite, an astronomical observatory that observes over a billion stars (!!) to determine the brightness, distance, position, colors and motion of each. The other is LAMOST, the Large sky Area Multi-Object fiber Spectroscopic Telescope, a Chinese observatory that takes spectra of stars and can obtain their chemical composition. This last part is essential: when the Universe was young, it contained almost no heavy elements such as calcium and iron; these were later created into massive stars, which then exploded and seeded newly formed stars with these elements. The measurement of a star’s composition can then be used to determine its age by comparing it to proven physical models of how stars evolve over time.
But it’s not easy. A star’s internal processes can make aging difficult and introduce a lot of uncertainty. So astronomers in the new work targeted stars called sub-giants: Those who are just beginning to run out of hydrogen in their nuclei to fuse into helium. This is a relatively brief phase in the life of a star and can greatly reduce age uncertainty.
They observed a staggering quarter of a million subgiants in the Milky Way’s disk and, coupling the ages with star distances, discovered that the galaxy’s disk is a bit more complicated than previously thought.
We know that there are actually two discs: A thin disc embedded in a thick disc. I have already written about this:
There are actually two disks in the galaxy: a thick disk and a thin disk, with the thin disk right in the middle of the thick one. Imagine the galaxy is a two-layer cake with frosting between the layers. The icing is the thin disk and the cake sponge is the thick. If the two sponge layers are exactly the same height, then the galactic midplane would slice horizontally exactly through the center of the icing layer between them. To give you an idea of scale, if the cake is 10cm tall – the thickness from top to bottom of the thick disk – then to scale with the Milky Way the cake itself would be a circle of 5 meters in diameter [Note: We now know the disk is wider than when I first wrote this, so it would really be more like 6 meters across]! It is a very wide and flat cake. The layer of frosting in the middle representing the thin disc would be about 4cm thick, which is a thick layer of frosting! Also, at this scale, the Sun would be 1.3 meters from the center, about halfway from the center to the edge.
The thin disk measures about 1,000 light-years from top to bottom, and the thick disk just over twice that. The thick disc contains the oldest stars in general, and the thin disc the youngest. The thick disk has been thought for some time to have formed around 11 billion years ago, just under 3 billion years after the Big Bang.
The new research shows, however, that the thick disk is much older than that: it began forming 13 billion years ago, less than a billion years after the Big Bang. It’s amazing ! This means that the huge cloud of hydrogen and helium that collapsed to form our galaxy fell into place very quickly, quickly becoming a cohesive structure. The work also shows that the process of star formation continued for 6 billion years, a long time.
Another very interesting thing they found was that the age of the disc was strongly correlated to the amount of heavy elements it contained everywhere they looked in the disc, indicating that the gas used to make the stars was quite well mixed during this period. This in turn implies that the gas was very turbulent – turbulence is a very efficient way of mixing a fluid – so the heavier elements created in massive stars and sent back out into space when they went supernova were evenly distributed throughout the disc.
This work, if it proves to hold up, gives us a new chronology of the Milky Way. The thick disc began to form immediately, peaking around 11 billion years ago. It’s also at the same time that a massive galaxy, dubbed – seriously – the Gaia-Enceladus sausage, collided with our Milky Way, bringing lots of gas and stars to both the disc and the halo of the Milky Way. This clearly triggered a lot of star formation in the thick disk.
After that the disk stabilized, collisions of gas clouds within the disk caused them to approach the galactic midplane – an imaginary horizontal line bisecting the Milky Way that defines north and south. galactic south, much like Earth’s equator – forming the thin disc. This material then underwent star formation, the second episode in the galaxy’s stellar birth history. The Sun formed from this material in the thin disk, very close to the galactic midplane, and which we now know is about 55 light-years north of this line.
So while this may all sound a bit esoteric and technical, remember that what we’re trying to figure out here is nothing less than how we’ve become. The Sun formed as a result of this galactic collision, which affected the thick disk and partly created the thin disk – the same material that formed the planets, the Earth and you. Me. Everyone and everything you see around you.
You can think of astronomy as things happening above your head, but your head itself owes its existence to what’s going on up there. One of the main purposes of astronomy is to understand how.